Re: [PATCH 2/2] drm/dumb-buffers: document abuses in the UAPI document

[Thomas Zimmermann]

Hi

Am 26.02.26 um 17:06 schrieb Icenowy Zheng:
> 在 2026-02-26四的 17:01 +0100，Thomas Zimmermann写道：
> > Hi
> >
> > Am 26.02.26 um 07:37 schrieb Icenowy Zheng:
> > > Although the current DRM_IOCTL_MODE_CREATE_DUMB interface design
> > > only
> > > suits linear buffers, some KMS drivers abuse it for AFBC
> > > framebuffers.
> > >
> > > Mention it in the interface document, and clarifying that it's not
> > > recommended.
> > >
> > > Signed-off-by: Icenowy Zheng <zhengxingda@xxxxxxxxxxx>
> > > ---
> > >    include/uapi/drm/drm_mode.h | 4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/include/uapi/drm/drm_mode.h
> > > b/include/uapi/drm/drm_mode.h
> > > index cbbbfc1dfe2b8..02b6fa7ade355 100644
> > > --- a/include/uapi/drm/drm_mode.h
> > > +++ b/include/uapi/drm/drm_mode.h
> > > @@ -1256,6 +1256,10 @@ struct drm_mode_crtc_page_flip_target {
> > Since you're at it, you should move the line at [1] right here as the
> > first sentence of this paragraph. Then continue with 'Do not
> > attempt'.
> >
> > With that change, the previous paragraphs refer to pixel formats and
> > the
> > final one refers to framebuffer layout.
> >
> >
> > >     * Do not attempt to allocate anything but linear framebuffer
> > > memory
> > >     * with single-plane RGB data. Allocation of other framebuffer
> > >     * layouts requires dedicated ioctls in the respective DRM
> > > driver.
> > > + * There exists drivers exploiting this interface for framebuffers
> > 'exists user-space software'
> Ah the drivers seem to be also criminal to accept this kind of usage,
> it's why I thought about writing "drivers" here.

Each driver implements dumb buffers on top of its own memory manager. 
They are complicit in the sense that they often never tested against the 
limitations of the dumb-buffer ioctl. But OTOH the limitations where 
never written down clearly. Only very recently did we add the code and 
documentation to clarify dumb-buffer semantics. [1] And not all drivers 
have been updated yet.

Therefore several user-space programs used dumb-buffers in an 
"out-of-scope fashion" and got away with it. The comments in 
drm_mode_size_dumb() list the known exceptions.

Best regards
Thomas

[1] https://lore.kernel.org/all/20250821081918.79786-3-tzimmermann@xxxxxxx/

> Thanks
> Icenowy
>
> > > + * with modifiers other than DRM_FORMAT_MOD_LINEAR, but this is
> > Rather 'with non-linear layout'.  The reference to MOD_LINEAR seems
> > out
> > of place here.
> >
> > > + * incorrect usage with undefined behavior, and should not be
> > > + * replicated.
> > Best regards
> > Thomas
> >
> > [1]
> > https://elixir.bootlin.com/linux/v6.19/source/include/uapi/drm/drm_mode.h#L1216
> >
> > >     */
> > >    struct drm_mode_create_dumb {
> > >    	__u32 height;

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)